Relief images can be provided and used in various articles for many different purposes. For example, the electronics, display, and energy industries rely on the formation of coatings and patterns of conductive materials to form circuits on organic and inorganic substrates. Such coatings and patterns are often provided using relief imaging methods and relief image forming elements. There is also need for means to provide fine wiring in various articles.
Flexography is a method of printing that is commonly used for high-volume printing runs. It is usually employed for printing on a variety of soft or easily deformed materials including but not limited to, paper, paperboard stock, corrugated board, polymeric films, fabrics, metal foils, glass, glass-coated materials, flexible glass materials, and laminates of multiple materials. Coarse surfaces and stretchable polymeric films are economically printed using flexography.
Flexographic printing members are sometimes known as “relief” printing members (for example, relief-containing printing plates, printing sleeves, or printing cylinders) and are provided with raised relief images onto which ink is applied for application to a printable material. While the raised relief images are inked, the relief “floor” should remain free of ink. The flexographic printing precursors are generally supplied with one or more imageable layers that can be disposed over a backing layer or substrate. Flexographic printing also can be carried out using a flexographic printing cylinder or seamless sleeve having the desired relief image.
Flexographic printing members can be provided from flexographic printing precursors that can be “imaged in-the-round” (ITR) using either a photomask or laser-ablatable mask (LAM) over a photosensitive composition (layer), or they can be imaged by direct laser engraving (DLE) of a laser-engraveable composition (layer) that is not necessarily photosensitive.
Gravure or intaglio printing members are also relief printing members in which the image to be printed comprises depressions or recesses on the surface of the printing member, where the printing area is localized to the areas of depression that define the pattern or image. The process for using gravure or intaglio printing members is the reverse of flexographic relief printing wherein an image is raised above the floor of the flexographic printing member and the printing area is localized at the contact area of the top surface protrusions.
Laser ablation or laser engraving can be used effectively with an appropriate laser-engraveable precursor to form images for either of the above-mentioned printing processes.
Flexographic printing precursors having laser-ablatable layers are described for example in U.S. Pat. No. 5,719,009 (Fan) where precursors include a laser-ablatable mask layer over one or more photosensitive layers. This publication teaches the use of a developer to remove unreacted material from the photosensitive layer, the barrier layer, and non-ablated portions of the mask layer.
There has been a desire in the industry for a way to prepare flexographic printing members without the use of photosensitive layers that are cured using UV or actinic radiation and that require liquid processing to remove non-imaged composition and mask layers and that generate significant amount of liquid waste. Direct laser engraving of precursors to produce relief printing plates and stamps is known, but the need for relief image depths greater than 500 μm creates a considerable challenge when imaging speed is also an important commercial requirement. In contrast to laser ablation of mask layers that require low to moderate energy lasers and fluence, direct engraving of a relief-forming layer requires much higher energy and fluence. A laser-engraveable layer must also exhibit appropriate physical and chemical properties to achieve “clean” and rapid laser engraving (high sensitivity) so that the resulting printed images have excellent resolution and durability.
A number of elastomeric systems have been described for construction of laser-engraveable flexographic printing precursors. For example, U.S. Pat. No. 6,223,655 (Shanbaum et al.) describes the use of a mixture of epoxidized natural rubber and natural rubber in a laser-engraveable composition. Engraving of a rubber is also described by S. E. Nielsen in Polymer Testing 3 (1983) pp. 303-310.
U.S. Pat. No. 4,934,267 (Hashimito) describes the use of a natural or synthetic rubber, or mixtures of both, such as acrylonitrile-butadiene, styrene-butadiene and chloroprene rubbers, on a textile support. “Laser Engraving of Rubbers—The Influence of Fillers” by W.Kern et al., October 1997, pp. 710-715 (Rohstoffe Und Anwendendunghen) describes the use of natural rubber, nitrile rubber (NBR), ethylene-propylene-diene terpolymer (EPDM), and styrene-butadiene copolymer (SBR) for laser engraving.
U.S. Pat. No. 5,798,202 (Cushner et al.) describes the use of reinforced block copolymers incorporating carbon black in a layer that is UV cured and remains thermoplastic. Such block copolymers are used in many commercial UV-sensitive flexographic printing plate precursors. As pointed out in U.S. Pat. No. 6,935,236 (Hiller et al.), such curing would be defective due to the high absorption of UV as it traverses through the thick imageable layer. Although many polymers are suggested for this use in the literature, only extremely flexible elastomers have been used commercially because flexographic layers that are many millimeters thick must be designed to be bent around a printing cylinder and secured with temporary bonding tape and both must be removable after printing.
U.S. Pat. No. 6,776,095 (Telser et al.) describes elastomers including an EPDM rubber and U.S. Pat. No. 6,913,869 (Leinenbach et al.) describes the use of an EPDM rubber for the production of flexographic printing plates having a flexible metal support. U.S. Pat. No. 7,223,524 (Hiller et al.) describes the use of a natural rubber with highly conductive carbon blacks. U.S. Pat. No. 7,290,487 (Hiller et al.) lists suitable hydrophobic elastomers with inert plasticizers.
An increased need for higher quality flexographic printing precursors for laser engraving has highlighted the need to solve performance problems that were of less importance when quality demands were less stringent. However, it has been especially difficult to simultaneously improve the flexographic printing precursor in various properties because a change that can solve one problem can worsen or cause another problem.
For example, the rate of imaging, edge sharpness, and cleanliness of the laser-engraved image features are now important considerations in laser engraving of flexographic printing precursors and can be critical parameters for high resolution printing performance. Although U.S. Pat. No. 7,290,487 (Hiller et al.) describes the use of hydrophobic elastomers for laser-engraving, such elastomers may be incompatible with many radiation-absorbers, providing defective engraved features. There remains a need to provide a laser-engraveable composition that provides sharp defect-free engraved image features.
Direct laser engraving has also been used to pattern various surfaces as described in U.S. Patent Application Publication 2011/0086204 (Wohl, Jr. et al.).
There is an increasing need to control the wetting properties of the laser-engraveable elements to enable controlled ink wetting of the laser-engraved elements and controlled ink separation and deposition from the laser-engraved element to suitable receiver materials.
U.S. Patent 2010/0151387 describes the use of adding low molecular weight fluorinated acrylates or methacrylates to a photosensitive printing plate to modify the wetting properties of the plate. However, these polymers do not provide for the performance properties required for a laser-engraveable printing element with differentiated ink wetting and release properties.
There continues to be a need to improve the sensitivity, manufacturability, and performance of laser-engraveable flexographic printing precursors (or other patternable elements) using laser-engraveable compositions having suitable physical and chemical properties. There is a desire to improve sensitivity, to improve selectivity of ink wetting and transfer, to reduce imaging time, and to increase the throughput of an imaging engraving apparatus. Also, there is a desire to achieve flexographic printing plate or other patternable elements that will provide relief images with good quality solid areas and dot reproduction even when printing is performed at high speeds.